1. Field of the Invention
The present invention pertains to fluids for use in the drilling and completion of oil and gas wells. More specifically, the present invention relates to fluids which form a buffer between and prevent the mixing of various fluids used in the drilling and completion of oil and gas wells. The fluids will be hereinafter referred to as "spacer fluids".
2. Description of the Prior Art
In rotary drilling of wells, a drilling fluid is usually circulated down the drill string and back up the annulus between the drill string and the wellbore face. The drilling fluid can contain many different chemicals, but will most often contain a viscosifier, such as bentonite. When a casing string or liner is to be cemented into the wellbore, any drilling fluid and remnants of the viscosifier present in the wellbore are preferably removed to aid the bonding of the cement between the casing string or liner and the wellbore. In removing this drilling fluid from the wellbore and to clean the annulus, a wash or spacer fluid can be introduced ahead of a cement slurry.
Spacer fluids are conventionally used in cementing operations related to well completion in the following manner. Drilling fluids and cement slurries are typically chemically incompatible fluids which undergo severe gelation or flocculation if allowed to come into contact. Thus, drilling fluid must be removed from the wellbore annulus immediately prior to cement slurry placement. Spacer fluids are pumped between the drilling fluid and the cement slurry to form a buffer and prevent the drilling fluid and the cement slurry from coming into contact.
Spacer fluids should also possess certain rheological tendencies, such as turbulent flow at lower shear rates, which assist in granular solids removal and which encourage the removal of the drilling fluid filter cake from the walls of the well. Indeed, a common cause of failure in primary cementing is the incomplete displacement of drilling fluids which results in the development of mud filled channels in the cement. These mud filled channels may be opened during well production permitting the vertical migration of oil and gas behind the casing.
Conventional spacer fluids are typically composed of an aqueous base fluid and a weighting agent. The weighting agent is included in the composition to increase the density of the spacer fluid to a desired value and to increase the erosion effect of the spacer fluid on the filter cake clinging to the walls of the formation.
The fundamental properties of the aqueous base fluid are typically particle stability in suspension (anti-settling properties), fluid-loss control, favorable rheology, and compatibility with drilling fluids and cement slurries. These properties are directly related to the composition of the spacer fluid.
Consequently, conventional spacer base fluids include at least: i) an anti-settling agent; ii) a fluid loss controlling agent; iii) a dispersing agent; and iv) a surfactant for obtaining a waterwetted surface to aid in cement bonding. As noted above, the final composition of conventional spacer fluids is obtained by adding a weighting agent to the aqueous base fluid.
The anti-settling agent, fluid loss controlling agent and dispersing agent may be constituted by a single component of the composition or by plural components of the composition. In addition, a single component may function as more than one of the above mentioned agents. The agents typically are soluble or dispersible in water.
Depending on the water available on site and on the geological strata encountered, the aqueous base fluid typically includes fresh water, sea water, brine, or an aqueous composition containing one or more dissolved salts, such as NaCl, KCl, MgCl.sub.2 and CaCl.sub.2. The spacer base fluid, however, must retain its above mentioned fundamental properties at all possible salt concentrations.
Spacer fluids are conventionally used over a wide temperature range spanning from the relatively cool surface temperature to the bottom hole circulating temperature (BHCT) which may reach 200.degree. C. or greater in the case of geothermal wells.
The term "anti-settling properties" refers to the capacity of the spacer fluid to keep the weighting agent particles in stable suspension throughout the cementing operation which may typically last for 1 to 4 hours or longer.
A spacer fluid is considered to have good fluid loss control properties if the fluid loss measured according to API Specification 10, Appendix F is less than 100 ml/30 min, and excellent if the fluid loss is less than 50 ml/30 min.
Favorable rheology of a spacer fluid implies that the fluid has minimum friction pressure while maintaining adequate suspension of solids.
Since the spacer fluid is to be pumped between the drilling fluid and the cement slurry for removing and replacing the drilling fluid in the well annulus, it is very important that the spacer fluid be as compatible as possible with both the drilling fluid and the cement slurry. Such twofold compatibility in the past has been quite difficult to obtain, primarily because the drilling fluid and the cement slurry are incompatible fluids.
The compatibility of a spacer fluid with a drilling fluid and a cement slurry is determined in the laboratory by studying the viscosity of binary or ternary mixtures of spacer fluid with drilling fluid and/or cement slurry varying over the range of 0 to 100% by volume for each component of the mixture.
The compatibility of the spacer fluid with the drilling fluid and the cement slurry is considered to be excellent if the viscosity of a mixture of the spacer fluid and the drilling fluid or the cement slurry at a given shear rate and temperature is equal to or less than the viscosity of the more viscous component of the mixture at the same shear rate and temperature. Likewise, the viscosity of a mixture of all three components is considered to be excellent if it is less than or equal to the viscosity of the most viscous component at the same shear rate and temperature.
Conventional spacer compositions, however, do not usually demonstrate good compatibility with mixtures of drilling fluids and cements while simultaneously presenting good rheological, fluid loss controlling and anti-settling properties over the entire range of shear rates and temperatures normally encountered in oil field services.
The present invention has been developed with a view to providing improved spacer fluids that have enhanced compatibility with drilling fluids and cement slurries, that create and effectively maintain a buffer zone between a drilling fluid and a cement slurry even when all three components become commingled, and that enhance the bond between the cement and the surfaces of the borehole and casing.